What are the things that are prohibited during fasting?

A fasting person must not vitiate his fast and lose its reward. He must restrain himself [both physically and mentally] from doing or thinking of anything that incurs the wrath of God the Almighty and which forfeits his fast. These include:
- Slander.
- Tale bearing.
- Gossiping.
- Looking at what God has prohibited.
- Quarrelling and discord.
- Severing ties of kinship.
Abu Huraira narrated that the Prophet (peace and blessings be upon him) said: "It may be that a fasting person may not receive anything from his fast except hunger, and it may be that a person praying qiyaam [night vigil prayer] may not receive anything in recompense except for staying up late" [recorded by Al-Nisa`i, Ibn Majah, and Ahmed. Ibn Khuzaimah, Ibn Habban and Al-Hakem declared it authentic].
Anything that eliminates or diminishes piety eliminates or diminishes rewards because fasting was legislated to attain piety. God the Almighty says: “O you who believe! Fasting is prescribed to you as it was prescribed to those before you that you may (learn) self-restraint” [2: 183].
And God the Almighty knows best.